1. Field of the Invention
The present invention relates to radar target detection methods, and particularly to a target azimuth detection method and a radar apparatus using the detection method.
2. Description of the Related Art
Various radar apparatuses mounted in front portions of vehicles to detect targets have been developed. Such radar apparatuses transmit transmission waves towards predetermined detection areas including areas extending forward of the vehicles and receive reflection waves reflected from targets located within the detection areas. In a type of radar apparatus which has been in widespread use, at least either a transmitting antenna or a receiving antenna is configured as an array antenna composed of a plurality of antenna elements, and an azimuth of a target is detected on the basis of signals of a beam formed at different azimuths.
Spacing between antenna elements constituting an array antenna may simultaneously constitute an advantage and a disadvantage of a radar apparatus. For example, when a distance between adjacent antenna elements is equal to or less than a half wavelength of a transmission signal, no grating lobes are generated. This allows precise detection of a target azimuth while decreasing the azimuth resolution. On the other hand, when the inter-element spacing is greater than a half wavelength of the transmission signal, an array antenna with a large aperture can be realized with a small number of antenna elements. This permits a radar apparatus to have a high azimuth resolution at low cost while resulting in generation of grating lobes.
To overcome the above disadvantage, several techniques have been developed.
For example, Japanese Unexamined Patent Application Publication No. 11-231040 discloses a radar in which an azimuth at which a directional intensity of a transmission/reception signal in a directivity pattern of an array antenna is null matches an azimuth at which grating lobes are generated.
In addition, a radar disclosed in Japanese Unexamined Patent Application Publication No. 2000-258524 includes a first antenna capable of generating a beam in which intervals between grating lobes are small and a second antenna capable of generating a beam in which intervals between grating lobes are large. In this radar, the first antenna is used for detection of a target located at a distant position, and the second antenna is used for detection of a target located at a near position.
Further, a radar device disclosed in Japanese Unexamined Patent Application Publication No. 2005-164370 utilizes the fact that azimuth angles at which grating lobes are generated depends on a transmission frequency. In this radar device, a transmission frequency is changed so that angles at which grating lobes are generated are changed with time.